The effect of the selective dopamine receptor agonist fenoldopam on renal perfusion, renal function and cardiac function and cardiac hemodynamics will be assessed in 30 patients who demonstrate low cardiac output syndrome (LCOS). Ten patients without low cardiac output syndrome following open heart operation will serve as controls. Fenoldopam is an investigational agent that is 6x as potent as dopamine in causing renal vasodilatation. It should be an ideal agent in reversing the renal hypoperfusion thought to play a significant role in acute renal failure following post-cardiotomy low output syndrome. Fenoldopam also causes systemic vasodilatation and should provide effective afterload reduction.